LIASSIC ICHTHYOSAURS. 73 



In the pelvic fin the femur is longer in proportion to its breadth, and the distal 

 expansion is relatively greater than in the humeinis. The tibial ossicle of the three 

 tarsals has an emarginate tibial border ; the corresponding ossicle or phalanx of the 

 second and third series shows the same character. 



d. Ichthyosaurus platyodon, Ch. Tab. XX, figs. 4, 4' ; Tab. XXVII, figs. 1, 2, 3. 



The skull of Ichthyosaurus platyodon (Tab. XXVII, fig. 2), is somewhat longer in 

 proportion to the trunk than in Ich. communis. Taking as the trunk the extent of the 

 vertebral column to the pelvic arch, such extent includes, in the subject of fig. 1, one. 

 length and a half of the skull, while in Ich. communis (Tab. XXIV, fig. 1) it includes 

 rather less than two lengths, and, in Ich. intermedins (Tab. XXVI, fig. 1) rather more. 



The skull ol Ich. platyodon is longer in proportion to its breadth than in Ich. inter- 

 medius. The jaws are stronger from the greater relative depth of the mandible and 

 the less gradual attenuation to the rostral extremity. The orbit is of a full elliptic 

 shape, with less approach to the circular, than in Ich. breviceps (Tab. XXV, fig. 1). It 

 is relatively less than in the long- and slender-snouted kinds. The length of the rostrum 

 anterior to the orbit is three and a half times the longitudinal diameter of that cavity. 



The osseous circumpupillary ring includes thirteen sclerotic plates in the subject 

 of Tab. XXVII, fig. 2. The surangular (ib., fig. 2, 30) disappears betw^een the 

 dentary (33) and angular (31). This element similarly disappears in a pointed form 

 between the dentary (33) and splenial (32), beyond the midlength of the ramus. 



A dental characteristic of the present species is that which suggested to Conybeare 

 the name platyodon ; the smooth enamelled crown (Tab. XX, figs, 4, 4') being subcom- 

 pressed, sharp-edged, and pointed ; the longitudinal grooves of the tumid cement-clad 

 root are soon lost upon the coronal base. I have counted 45 — 45 of these teeth in an 

 upper jaw, and 40 — 40 in a lower jaw, and have noted that the crowns are more often 

 snapped ofi" than in the smaller species, which may be indicative of the greater violence 

 with which they have been used. 



From the occiput to the iliac bones there are forty-five vertebr£e ; thence to near the 

 end of the tail may be counted seventy-five vertebrae ; the total number in the skeleton 

 probably somewhat exceeded 120. One of these vertebrae, from the hinder half of the 

 abdomen, is figured (in the inverted position with the neural surface downwards) in 

 Home's Memoir of 1816.^ The neui'al spines are thicker, shorter, and more rounded 

 superiorly than in Ich. intermedius or Ich. commmiis. The zygapophyses, especially the 



^ ' Phil. Trans.,' mdcccxvi, pi. siv. 



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